K_sp = values: BaCO_3 = 2.0 times 10^-10, SrCO_3 = 5.4 times 10^-10, CaCO_3 = -3
ID: 506883 • Letter: K
Question
K_sp = values: BaCO_3 = 2.0 times 10^-10, SrCO_3 = 5.4 times 10^-10, CaCO_3 = -3.3 times 10^-9, MgCO_3 = 3.5 times 10^-8 Ba(OH)_2, Ca(OH)_2 Sr(OH)_2 all have K_sp values > 10^-6. Mg(OH)_2 = 6.3 times 10^-10 Review the procedure in GS4. The separation of Ca^2+, Sr^2+, and Ba^2+ as carbonates must be done under weak basic conditions and moderate carbonate concentration. a. First describe how the concentration of [OH^+] is limited so that Mg(OH)_2 does not precipitate. Use chemical equations to support your answer. b. Now explain how limiting the [OH] concentration also limits the concentration of [CO_3^2].Explanation / Answer
(2)
(a)
In order to keep the [OH-] mild, the pH has to be kept closer to 7.
This is achieved by adding a weak base which maintains the following eqilibrium in the solution:
MOH <---> M+ + OH-
A buffer made with a weak base (high value of Kb) and its salt with a strong acid will be essential for maintaining the desired [OH-]
(b)
Reaction occuring is:
CO32- + H2O <---> HCO3- + OH-
Because of the above equilibrium between the carbonate ion and hydroxide ion, the carbonate ion conc. is also under control as long as [OH-] is kept under controlled limit.
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